Force by price, upkeep and vunerability... I don't think this can be regarded as a "hardlimit" of any kind. It's a gameplay "limit". The "manipulating game mechanics" - as you put it - means exactly the same IMO.
Sorry, I saw the word "force" and I guess I panicked

Not really the way I'd usually use the word "forced"... after all, I can still go all-capship, its just that that approach sucks.
1) If you increase the overall speed/turning speed of the smallest ships (i.e., give fighters a big speed boost, frigates a slightly smaller speed boost, and cruisers and bombers smaller still, but leave cap ship speed alone) you can increase the tactical value of the less often used ships. The goal being that in a scenario of all cap ships, the frigates could run circles around them continually attacking from behind. The fighter speed increase would help mitigate the use of bombers since if two fleets started at any appreciable distance, the fighters could intercept the bombers before they could release their payloads. In non-combat situations, the increased speed is also quite useful for exploration and getting reinforcements to systems under siege. It also makes guerrilla warfare a possible tactical option as frigate fleets could do hit-and-runs.
I don't know about the rest of it, but giving smaller vessels a speed boost would also make them more useful for nodal defenses -- park them someplace, and have the fleet jump out to respond to attacks on neighboring systems. As things stand, there's no difference between using a capship or frigates for this, except that capships are a lot more powerful once they arrive.