You can only do that with one thing per faction. The way it works is you have a tech require a certain faction ID, which then means you're essentially allowed to make one choice. It can be a choice with 2 options, or 8, but it can still only be one choice. Also, if you were to do something like that you're using a lot of memory for ships in redundant roles. It would essentially always have to be adding new ships to existing roles, rather than making it be choices between existing stuff (you need some options within the game beyond just one choice that then locks you in forever, after all; if you choice a tankier ship ie the Praetor at one point, it shouldn't mean you can never go for an offensive option at all, ala the World Devastator; people also would get upset if the unit rosters were limited to effectively 5 frigates, 3 cruisers and 3 capital ships, for example, even if it's a result of having the choice between 11/11/7 or something). When developing playstyles, it's more important to try to nail that down between the factions, rather than diluting that by trying to make 2 hard-defined styles within the same faction, and then doing that 5 times. You're then working to define 10+ overall playstyles and it dilutes everything. With the memory usage it also limits the amount of actual content you can have. This is pretty important in Sins, which requires memory modification (LAA) to run mods in the first place; if you did that sort of branching and had even 1.5x the ship numbers within the same faction, you're sacrificng a lot of overall diversity between factions, since you can't use those resources in game or development-wise for a full faction that can have their own very distinct playstyle options to get what amounts to a tiny amount of diversity within it.
To use a more direct example, if we give you the choice between the Nebulon-B and the Corona as the NR or the IPV/Tartan/Lancer for the IR, are those minor stat differences really a more interesting gameplay choice than what we'd get out of using that space for something like the Sh'ner and Fw'Sen, which while as ships provide some similar stats but as a delivery mechanism can provide for different playstyles within the context of what the Ssi-Ruuvi as a whole can offer (Entechment, etc)? Basically, when it comes to roles and ship types within those roles, the greater variation isn't in the minor stat differences within a faction (because you're ultimately going to be using those ships the same way), rather it's how those roles are emphasized within the faction.