I think you can make a custom offer on Fiverr Anywhere. If a seller contacts you, you can provide a discounted rate via a custom offer in the inbox.
In your gig page, you could say something like “If you are a Fiverr seller and wish to collaborate, message me to get the ball rolling” or something like that.
What you cannot do is message other sellers (or any user) and offer your services. That’s spamming and breaks the TOS.
I don’t think seller collaboration is a bad thing, but I do suggest you be careful. I was collabing with a seller and they frequently asked for much more than I wished to provide, when I was already deeply discounting my prices. It’s also difficult to communicate with the original buyer if a seller is acting as a middle man. If you find a seller who would like to collaborate with you, perhaps you can recommend each others gigs or something to buyers. HOWEVER, you can’t buy from a seller who has bought from you or it looks like review exchange (which a lot of newer accounts try). All interactions must be one-way, I believe.
If you are worried that an action might break the ToS… first read the ToS, and then search and then ask on the forums or Customer Support if it’s still unclear.