aquathon advice?

For your first one, I'd pace the 5k conservatively - like a 10k pace. Settle into a hard, comfortable swim, then all out on the final 3k.

Sounds like fun

If you can warm up in the pool, I would do so.  Do some run warm-up either way.  At least do a few short strides at around race pace, after your warm-up jog.

I would go out like you are running a 10k--should be 'comfortable but hard' through the first 5k.  Your swim should be be done similarly (i.e., perhaps at a pace you would expect to be able to hold for a 1000m stand-alone TT).  Last 3k is as hard as you can manage.  Try to hold close to your first leg pace (but effort will feel much harder this time)--or open it up and go faster if you are able.

Oh, and definitely the tri suit or something similar.  No loose clothing at all.

Always look at temperature for during the race, not the high.

The tri-suit should be more than enough. You'll get quite warm in a run event with substantially lower temperatures. The swim is in a pool, so will not be an issue.

I'd get in a little bit of swimming to loosen up the shoulders. And also whatever running warm-up I'd normally do for a running event, as that is what you're starting with. Typically that is 10-15 min of easy running and some strides up to about expected race pace.

How have you gauged your effort for harder efforts? I'd be close to, but just behind, open pace for each segment. Or just over threshold pacing.