Well my rtek 1.7 chip is here ive bought some used 720cc secondaries off a friend of mine can I just install them no worries or is there more to this, they came out of a s5 just like mine so? Should there be any problems or are they "plug and play"

 

If the 750s are s5 and yours are too, plug them in and walk away.

I'm not to familiar with rteks, but i think theres a setting for injectors you have to choose so it knows what size its controlling

 

The Rtek 1.7 has a set Injector table..so you can only use what size injectors the Chip allows for.
The RTEK 2.1 however can use a variety of injector setups..depending on if the variance is allowable..
like 750/1000..that is a variance of 25 per cent..Y0u can't go TOO overboard and have the car function 100%...550/1600 is a Bitch..

If the 720's have the same connector then YES you can use them.
you need to check the impedance of injector to make sure that ALL injectors are the same.The S5 uses HI impedance.
NOW if the injectors do NOT have the same connector but the same impedance then you can change the connector at the harness.