Unless they're all written by the same composer, I'd say the resemblances are purely coincidental. Megaman music isn't exactly known for its recurring motifs.
(Although once again, the NES track blows the rest of them out of the water.)
Wha? Mega Man has had plenty of remixes. OK, not in Castlevania quantities, I admit. But look at the number of times the stage introduction from the original game was used (2, 4, 7, 8, R&F, X1..)
Some more examples:
-Guts Man Stage, Snake Man Stage->Robot Museum
-That one stage complete theme that's in a lot of the games you know the one I mean
-Dr. Wily Boss Battle->Shadow Devil
-Mega Man Ending->Mega Man 2 Intro, Mega Man 4 Credits
-Zero's Theme 2->American X5 Intro, Zero cutscene from X6
-Dr. Light's Theme->Dr. Lights Theme (X5)
-Sigma Boss Battle->Eehh, it was in X5 also.
-Sigma Battle->Sigma Battle1 (X2), Final Battle(X6)
-Sigma Battle 2->..Sigma Battle 2(X2)
-Bubble Crab->Tidal Whale/Makkoin/Duff M-aw screw that.
And within the Zero Series is pretty obvious stuff..
-Infiltration->Infiltration II (Incidentally.. the Remastered version of II? Winamp says I've played this tune 113 times recently. Great, great tune. Love it. *plug*)
-Neo Arcadia->Neo Arcadia II, Neo Arcadia III
-For Endless Fight->For Endless Fight II, For Endless Fight III
-Crash I through Crash IV.
-Sand Triangle->Sand Triangle II
-Departure->Awakening Will (It counts if it's in the same game, right? No? OK, that makes things easier on me then. *whew*)
-Cannon Ball->Cannon Ball - Hard Revenge
Also, I'm fairly certain different composers have remixed other composers works before. Example, example..
Ah yes, Theme of Zero from Mega Man X1 by Setsuo Yamamoto, remixed by Ippou Yamada in Mega Man Zero.
Also note I actually haven't gotten around to X7 or X8 so if I missed any from those.. yeah.