FSD booster and SRP don't have human analogues. Unlock these as soon as you can, they're basically must have, especially FSD booster.
Guardian PP and PD are bad pre-3.0 rolls of overcharged PP and charge-enhanced PD. You can use them if you can't get the proper stuff right away, but as soon as you can, dump them and never come back.
Guardian HRP is somewhat useful if you actively fight Thargoids. Install the smallest one for a bit of caustic resistance.
Guardian MRP is slightly better than human's in terms of integrity, but it comes at price of energy consumption. If you have spare energy, go for it. If you haven't, don't.
Guardian weapons are must have for fighting against Thargoid Interceptors. If you're not interested, don't bother.
Guardian fighters are nice, but kinda useless against human ships. Also, they look really cool. You can unlock the Trident - it is slightly less useless than others, however proper fighters like plasma-wielding GU-97 or MC-Condor would mop the floor with it.