Welcome to the very reason why FTB as a modpack exists!
When mod authors make their mods, they give their items more or less arbitrary IDs within the range that is allowed by Minecraft. With the obvious result that putting many mods together is completely impossible because they will be randomly tripping over another. What the FTB team does for you - and the reason for there being a team in the first place - is look at every single mod in the pack and completely reassign all item IDs in such a fashion that all mods still work, none conflicts with another, and all mods still have a little bit of space around them to grow, AND that there's room for new mods in the future as well, which may or may not exist and are of unknown design.
If you want to add a mod that has ID conflicts, you need to do the following:
1.) Find out the IDs used by Tropicraft from the config file.
2.) Look at the crashlog and find out which mod conflicts with Tropicraft. In this case it is Xycraft.
3.) Look at Xycraft. Compare it to Tropicraft. Find all conflicting IDs.
4.) To reassign an item ID, find a gap between the mods somehwere that has enough space. You can for example look at NEI where the items of one mod stop and the items of the next begin. Or you can look through the config files of all the mods in the pack.
5.) Do not, I repeat, do not reassign anything from Xycraft or any other FTB mod, you'll likely only break things further. Reassign IDs only from Tropicraft.
6.) After you have reassigned all conflicting IDs you could find, try starting again. It either works, or it will throw a different conflict. Repeat steps 2 to 5 until you have no more conflicts.
Keep in mind that there is a difference between "block" IDs and "item" IDs. Specifically, blocks used in worldgen, such as ores, must have a block ID under 256. This is the single biggest bottleneck, and you might have to look extra carefully to find room.
Finally, note that I have never done this myself. I'm giving you the theory, not a proven how-to guide. If I were you I'd read up on ID handling in Minecraft first, and should you find someone who actually knows this stuff, discard all of my advice in favor of his