Trouble finishing my repertoire

I'm almost done completing a fun opening repertoire but i have no clue what to play against 1. c4 and especially 1. Nf3. I've been skimming through some ideas where i might play a reverse grand prix or sicilian against the english but im still not sure as some lines in the reverse gp look bad. And im really having trouble against the reti. I can probably go for a Benko if 2. c4 which is a system im good at but idk what to play if 2. g3 or something else. Obviously, im not well versed at these openings so an overview would really help me

Do not worry about openings.
You do not lose games are on moves 1-2, you lose games in moves 10-20.
Against 1 c4 or 1 Nf3 you usually can play the same you play against 1 d4 by transposition.
Reverse Grand Prix is not bad either especially when you play Grand Prixx as white.
Here is the textbook example of Reverse Grand Prix
www.chessgames.com/perl/chessgame?gid=1044658

Do the same principles in the Sicilian as white perfectly apply in playing the English as black? I don't have too much experience in it, but is being a tempo down fine for Black?

With black you have to be more precise exactly because you are a tempo down. With white you can make an inaccuracy worth a single tempo, with black you cannot. However, the Fischer game is a good example of how to play this with black.