Somewhat odd combination of size and ability is motivated by the needs of the remaining red rare creature slot. Does it seem too odd?

It seems fine to me. I agree I generally want a large trample creature or a source of repeatable non-tap damage, but I'm definitely happy to have both, and want both at least some of the time.

If you wanted, and the cycle permits, you might be able to tweak the abilities to feel slightly more in sync, eg. matching the power to the damage dealt, or having the damage dealt when attacking, although I'm not sure.

I added an extra rare slot to each colour (removing the rare land slots that I wasn't using). So now I could split this up into two cards if I wanted.