Solo campaigns are my preferred game type, so I will try to give whatever guidance I can on the subject.
First off, when I am playing a solo campaign I prefer to run it as an exploration campaign where the campaign map is dynamically generated as the empire's explore. This makes it easier to setup the game, as you don't have to pre-generate a map and system statistics for each star system, and also makes the game more exciting as you don't know what you'll find in the next star system. That keeps the game fresh and interesting and prevents you from doing long-term mental scheming that can skew a game in favor of a single power, a scenario that can happen if your map is already generated and you know exactly where every resource, inhabited system, and calamity on the map is.
There are probably two different starting conditions you can use for solo exploration campaigns. The first is to start with a single empire emerging from their homeworld (or homeworld plus a few other smaller colonies, if you want them to be a bit more diversified at the start of the game). The second is to start with 3-4 empires on the map to run from the start of the game. I would recommend a one-empire start for your first games just because it gives you less variables to deal with at the start of the game, but for more balanced solo campaign scenarios it is better to start with a few player empires spread across the map at the start of the campaign so that you get more balanced exploration results and have a better chance of forming major power blocs to make the diplomatic and political aspects of the campaign more interesting once they come in contact with each other.
As you explore, you'll make empire activation checks in each new system to see if you run across a new empire. I would recommend not rolling these for the systems within 2 jumps of your starting empires' home systems, however, because that can lead to powers being too close to one another at the start of the game. But beyond that limitation it should be fair game to run into a new empire.
When adding new empires to the game, you do have to balance the results of the activation rolls against what is actually going to be fun to play. I have had a few solo campaigns die because I ended up generating huge new empires. The act of setting them up to play ended up killing the momentum in these campaigns. If you end up running into an empire that is significantly larger than any of your player empires, it is probably best to reduce their territories to a more manageable level.
One of the more glaring problems with 1E is that there isn't a tech system to use to design new units, and the game relies on a Tech Year mechanism instead. There are a few workarounds for this, some that gel better with the rules than others. The first option is to embrace the Tech Year option wholeheartedly and draw up detailed force lists with units broken out by Tech Year. Using these pre-generated lists, and having a few spares available for new empires as they appear on the map, makes it easy to jump in and setup a new power because all of their unit stats are already available.
A variant of the above tech option is to give each empire a set of starting units and then allow them to design one new unit that is slightly better than the previous one each time they achieve a tech advance. I sometimes made this a percentage chance instead of automatic in my 1E games, with a cumulative +25% chance of a new design becoming available every Tech Year. The stats for the new unit have to be pulled out of the air, but since you are the only player in a solo campaign you can tweak and fix things on the fly if you end up designing a unit that is obviously too good or needs some mechanical tweaking because of a rules mistake and no one is going to complain.
Another tech option is to use the Starmada Edition rules and spreadsheet to design units and research technologies. You don't have to have any intention of ever playing Starmada, but the rules at least give you a more strict way of conducting research and designing combat units.
You can also create a home brew formula or archetype model for unit design in your campaign where you have baseline models that can be advanced in a tech tree fashion, with X number of points to spend between the abilities. Some of those points might be preassigned to the archetype with only a few leftover to be spent as the player wants. For example, a Destroyer-I might have the following stat block:
Construction Cost 3, Maintenance Cost 1/4, DV 1, CC 1, +5 Points
This gives you a decent little ship with some 5 points to split between the ship's stats. At his next tech advance, you could then give an empire a Destroyer-II that might increase its available points by +10% (min 1). The player could then prototype a new DD-II class that would have 6 points to spend. This versatile method allows you to design the following two ships at the DD-I level:
DV 2, AS 2, AF 0, CR 2, BC 0
DV 2, AS 1, AF 1, CR 1, BC 1
Not spectacular, but you have a bit of room to maneuver. The higher level archetypes then would cost more to build, maintain, and command, but they would also have better starting DV and more points to spend on abilities. It would just be a case of assembling the archetypes and determining how many points per level they would receive as a bonus. The cost of the advanced models would probably need to increase over time to balance out their capabilities, but that could be accomplished by comparing the number of points the units spent on abilities to a maintenance cost lookup chart.
Sorry for the tech derail, but that is usually a major point of consideration in a solo campaign.
The other thing that I would bring up is that it is often more interesting to run all of the empires in your solo campaign as NPEs even if they are "player empires" in every other sense of the word. I've found that applying the NPE diplomacy rules to all of the powers in a solo campaign makes things a bit more interesting because you'll never quite know what your own empire is going to try to do, and you end up spending intel points to try and influence relationships with other powers and maintain the level of relations that you would prefer. There is also nothing quite like having a string of bad rolls lead to war. If you only have one starting empire then you might not want to do this, just so you can retain more control over what your empire does, but it takes some of the guess work out of what an empire's priorities are and adds excitement to the game.
If you do start a solo campaign, don't hesitate to share the fruits of your labors with the rest of us and ask any additional questions that might come up as a result of the game. Half the fun of games like this is reading through the campaign diaries to see what happened with the rules and what kinds of adventures the empires had
