Well I can't say anything about Trinidad and RichFaces together, but even only Trinidad with Seam is quite tricky.
If you are using the 1.2.x branch with facelets, then you need to build facelets from VCS to make seam's validation work, because it needed a bug fix in facelets.
If you try to use Trinidad with JSF 1.2 RI, then again you need to build JSF from VCS because again validation will not work without a very recent change in the RI.
If OTOH you use Trinidad with MyFaces, then you cannot use server-side state saving, because Seam seems to trigger a bug in MyFaces' client-side state saving.
Additionally quite a couple of smaller issues make using Trinidad less than pleasurable for me.
Can you recommend RichFaces for my next project?
I am sorry to hear about the trouble with Trinidad. I guess the integration still isn't quite up to snuff. I guess I will have to figure something else out for my UI troubles.
I would recommend Rich Faces with Seam. It works out of the box and gives you a decent amount of components. Some places where it is lacking compared to Trinidad are the data table, layout, and selectMenu options. With the latest release (3.1) Rich Faces has addressed the data table features somewhat (added sortable columns and scrolling) but not select one or select many tables yet. Also there are no where near as many panel choices for layouts and no selectMenus. I really miss all the select Menus Trinidad offers.
On the other hand, Rich Faces "just works" with Seam, no extra hassle and you know its all supposed to work together.
Yes, we have (unsurprisingly!) been devoting most time to RichFaces integration. A solution to the Trinidad 1.2 woes may be to use the Trinidad 1.1 jars on a JSF 1.2 platform (should work just fine).
Also, there should be a new facelets release with that fix in soon now.