The reason people think this is ridiculous is because it cuts into the heart of the game. If you mean limit # of each ship (separate caps per ship type), fleet composition is absolutely vital to the strategy of this game. The purpose of fleet cap is to tie fleet strength to economic power, while allowing full control over what your fleet consists of.
If you just want less ships total, well, that's very easy to mod. Hell, there's an option that lowers fleet cap already in the game.
Since you don't play multiplayer (I don't either, to be honest), your best bet to fix something minor is to do it yourself. For example, I hate how planet exploration is practically worthless, and artifacts don't come up nearly often enough. So I mod out maps to have much higher rates of artifacts and resources- makes for more varied and interesting planets.
In this case, what you need to work on is changing fleet capacities- lower the base values/research boosts, and possibly increase the cap cost of all the "small" frigates- this will encourage using more powerful frigates, since you'll get more bank for your cap.
As for swarms being a problem- that's a tactical issue. It IS solvable, and it doesn't turn the game into a "shooter". Look around online for guides, don't forget to retreat to more advantageous positions, make use of abilities. Loosing a base or two happens.
Best of luck to your AI stomping!