You can't have a Link Team made of different units, unless the rules explicitely say otherwise.
So, Reverend Healer can be a part of Moira or Custodier Link, if I recall the Bakunin AVA / Link table correctly.
Kusanagi can be a part of Moira Link.
A Moira (including Kusanagi) can not be a part of a Custodier Link.
@ 2 - I'm not a Bakunin player, but one of my regular opponents is. There are three memorable options there:
1. Moderator Link. Cheap (good for Order generation), but they do not shoot too well. Still, with a Spitfire at a right range band, or Multi Sniper Rifle from far away (outranging your opponents) they can be pretty effective. Their Hacker is not stellar, but is cheap (important, if you intend to use the hacker primarily to fullfill the "Hacker required" condition for fielding REMs - and as Supportware programs work automatically, it doesn't matter who runs them).
2. Riot Grrl Link. Fast, inexpensive (for a HI) and resilient, they pack quite a punch. Especialyl up close (i.e. within optimum range for their Spitfire). Blitzens are very effective TAG-stoppers.
3. Moira Link. Actually, their main advantage is the ODD (-6 to hit hurts! Untill your opponent brings out an MSV L2 or a Direct Template weapon), though they carry quite a choice of weapons. But I'm not 100% sold on Medium Infantry...
@ 3 Sin Eaters are nasty - kinda Total Reaction REMs, but with mimetism, better eye and way more resilience. However, they can be dealt with - preferably from behind (by Airbone Deployed troops, Infiltrators and / or Camouflaged troops). Luckily for me, they are pretty expensive.
Do not underestimate the Mk12 variant - it may lack range and Burst, but it hits hard, and doesn't burden your SWC count.
Prowlers are good, but somehow I don't see them on table very often - Zeros are cheaper and more mobile.
@ 4 - defintiely Zonds. You can playtest Morlocks using any S2 models as proxies for the time being. That would be pretty difficult with REMs.
Morlocks are good, however, as troublemakers. And smoke is always nice to have!