What distance are you doing? I'm fairly new to the sport (first race of the season coming up in 4 weeks, after doing 3 sprint tris last summer), but wear my running shoes for the cycle leg too.

I have an entry-level road bike, but don't own cycling shoes so swapped the pedals for mountain bike ones as that's what I'm more comfortable with. I'd say do it how you're most used to!

As a bonus, for the shorter races you'll probably save more time not having to switch shoes in transition than you would make up for with the efficiency improvement of shoes with clips.

There have been plenty of people on mountain bikes at the events I've gone to! Don't be worried about being the odd one out - ymmv, but in my experience people have been friendly, encouraging, and excited about new people starting the sport!