[Hectic]

If the 150 puts you at 500 a 130 from the same company should out you at ~420fps
Laylax springs are in mps, provided you have good air seal and efficient barrel volume to cylinder volume their ratings are prety spot on. I run a vsr 10 with a 455 mm inner (full laylax internals basically) and a sp 150 gives me ~500fps
As for the spring size it's not that relevant. PDI sells some different sized springs and guides (for whatever reason) and they sell guide specific to those springs. If you have pdi parts you should order a pdi spring to go with the guide you selected. Otherwise stick with laylax . The laylax parts will fit with stock parts.
If you have some off brand (angle custom for example) you'll have to figure out what parts they are a clone of.
Also we need to know what l 96 you have because there are two versions AFAIK, one uses aps2 parts and one uses the type 96 parts. Then there are also the really off brand that use a mix and or include some Vsr10 parts.