can I not ally him in? I assumed you can because you can in WOTR and in LOME it says army list can ally to only certain other lists

A leader can only lead troops from his own list. Yes, you can ally him in, leading high elves. If you could just swap heroes around leading whoever they want, there would be no point in having the different lists or the concept of allies in the first place.

I assumed there was a similar rule to WOTR where 25% of your army can be allies but no more, my bad. In that case which high elf troops do you think I should take

i'm not sure whether to use them like galadhrim with spears or give them heavy armour and shields so they can take a lot of damage

The only Warriors (ie not Heroes) in the High Elf List, are High Elf Warriors. Take a couple with Elven Blades, with most of them having shields or spears. Give the front line heavy armour.

_________________
"I am the Flying Spagetti Monster. Thou shall have no other monsters before me"
-FSM.

how big do you think my warbands should be if these are the groups I'm taking

Glorfindel + high elves listed above

Rumil + galadhrim troops + bowmen +Guard of the Galadhrim Court

Haldir + same as above but more bows and less Guards

Take as many Warriors as you can in each Warband, about 10, 11 or 12 in each. You might want a 4th Warband too.

_________________
"I am the Flying Spagetti Monster. Thou shall have no other monsters before me"
-FSM.

well I'm only planning on a small force to begin with and then build up

Also I wondering is there any limitation for the number allies you can take as part of your force (apart from bow one I know that)

No, there is the 25% rule in WotR, and when SBG was in the thrall of LoME, there was a list of allies, and you could only ally with certain people eg, Isengard couldn't ally with Easterlings.

In SBG: Warbands, the only ally 'restriction' is that every Warband, from every allied contigent must be led by a hero. That is all. And the 33% bow limit etc.

_________________
"I am the Flying Spagetti Monster. Thou shall have no other monsters before me"
-FSM.

thanks just in my head had an image of using all my elves in a huge elf war host representing the 3 remaining kingdoms with all the Kings Guard, Noldor exiles, whatever the Thranduil upgrade is called and the galadhrim guard upgrades

High elves all come with heavy armour. The old rules let you pick any equipment, but the new profiles are more specific.