You have really quirky titles for your mashups, which I envy. Here's my 2 cents:
- Megalotrousle - This has a good blend of instruments. There are some clashing notes here and there, but the balancing mostly makes them not jarring. Well, all up to bar 24-26, where things get a bit...off. Like significantly. I do find it interesting that you pretty much retain Megalovania's chord progression, even when Bonetrousle kicks in. Wave 75 for track 3 works wonders here. I would have liked Bonetrousle in that mashup to have its "moment" rather than being mixed in with Megalovania's chord progression (which doesn't work). Particularly at bar 40; this is pretty much just Bonetrousle's theme playing here with a "Megalovania" feel coming from the way the notes are arranged as well as the percussion. Let that carry the "Megalovania", and try adjusting the chord progression to match how Bonetrousle is actually supposed to be played, notewise.
- Scorching Glamour - I'm not really convinced by this piece that these two tracks are meant to go together, except measure 24 to the loop end. And the way it loops is strange and isn't clicking for me.
- Thermal Vents and Waterfalls - Okay I think that these two tracks could go really well together in principle. Nice effort trying to make a triple metre song fit into a 4/4 grid though. I'm not hearing enough "Geothermal" though - to me, it just sounds like a remix of "Waterfall" with a "Geothermal" sound to it.
I personally don't think it's necessary to constantly overlay two themes together in a mashup/medley. Consider integrating some moments where one section of a selected track will simply flow to another, with minimal accompaniment from the other track just to keep the feel. I do hear this a bit in Megalotrousle, but I think there might have been more opportunities for this in the other mashups.