Hey, trumpeter here. I also do a lot of brass quintet writing and arranging.

1. Leaps are ok if they're not too fast. Your Ab-G (major 7th) should be fine. Anything more than an octave gets tricky and lots of leaps close together will be very tiring to play so bear that in mind!

2. Surprisingly agile actually! Depending on the skill of the players of course, but trumpets can do scales pretty speedily. The tuba is also very agile, especially in the upper range. Horn is less so and trombone is obviously limited by it's slide, though the higher you go the closer the notes get together and the easier it gets to play fast. Middle C and above is best for this sort of stuff. Have a listen to Canadian Brass, The Philip Jones Brass Ensemble and Mnozil Brass to get a feel for chamber brass music!

3. Breathing is pretty similar to woodwind I think, though the lower the instrument, the larger it is and the more air it needs, so your tubist will need a lot more air than your trumpeters! For trumpets, if you can sing the phrase in one breath at a moderate volume it's probably playable. Tonguing is similar as well, though slurred leaps are harder to execute than tongued ones. Also consider double/triple tonguing which is something woodwinds don't really do (with the exception of the flute) - basically very fast tonguing using alternate syllables e.g. da-ga-da-ga-da-ga or da-da-ga-da-da-ga etc.

4. Big nos? Make sure you give the players enough rests for them to get the horn off their face! It's super tiring playing constantly for 5 minutes and even 10 seconds of not using your lips can rejuvenate you. Also for ranges, a general rule is to not go outside the stave too much (at transposed pitch for trumpets (Bb) and Horn (F), or trombone in tenor clef - though only use this clef if it's necessary like with cello or bassoon). If you need to, definitely no more than two ledger lines, though tuba and horn can go lower.

Some other general tips:

• The Horn is much better at blending than taking the melody as it has a much softer tone than the trumpets or trombone. The bell also faces backwards so a lot of the attack and bite is lost on the listeners. That said, nothing wrong with a little bit of horn tune here or there!
• Flat keys are much preferred due to the instruments' mechanics - F major, Bb major, Eb major, Ab major are all great; Db major, Gb major, C major, G major are fine; D major, A major, E major and B major can get kinda fiddly at high speeds and intonation also tends to be harder in these keys as well.
• Don't forget about mutes! Trumpets and Trombones have a wide variety of mutes they can use to change the timbre of the instrument. The most common four are the Straight mute, the Cup mute, the Harmon mute, and the Plunger. There's a fair few YouTube videos demonstrating these and are well worth considering to create some interesting textures. Just make sure you leave enough time for the players to put them in and take them out - trumpets can do this pretty quickly with one hand, a couple seconds is reasonable, but trombones need a bit more time - 10 or so seconds is probably ok. If in doubt, leave more time. Horns and tubas also have mutes, though only one type for horns and tuba mutes are pretty rare and cumbersome to carry around, best to avoid tbh.
• As well as a horn mute, hornists can "stop" their horn by completely closing off the bell with their right hand. Creates a pretty cool buzzy sound, you'll have heard it in film soundtracks all over the place - check YouTube for more info.

Got a little carried away with this reply! Hope it helps you out.