Hughes has never performed on any of their records, neither has he played with them live. Having said that Chad first performance with Glenn on Soul Mover was after the last RHCP studio album.
I highly recomend you check them out, they are one of my favourite bands. Their early stuff is more punky and less to my personal taste, but it isn't without merit. When John Frusciante and Chad Smith joined and they released the album Mother's Milk they became hugely popular. That album is great, but the follow up Blood Sugar Sex Magik is better, it is often seen as their best album. Frusciante then left the band and Dave Navarro who plays on Soul Mover and She Moves Ghostly did one album with them called One Hot Minute, which is pretty good, but not as good.
They then almost broke up after running through lots of guitarists before Frusciante re-joined and they made Californication, which is great (but sometimes slightly over-rated IMO). They then made By The Way, which is my personal favourite, they used lots of different styles including more ambiant tones and progressive elements. Their mew album Stadium Arcadium is out next month.
Frusciante (who plays on Nights In White Satin and some songs on Music For The Devine) has also made several solo albums, which I can't recomend enough! He is my favourite singer-songwriter and his solo albums are experimental and totally different from the RHCP music, but I find them so involving and I really connect with them.