So here is Mike Ware's response to my query in case anyone is curious (I promise I'll now stop talking to my self)...

As you may have guessed, this is not the first time this question has
been asked, but it's a good one and deserves a sensible answer.

Since you have a copy of my 'Chrysotype Manual' (Bravo! Would that
more did!) then you can read my reasons for preferring ammonium iron
(III) oxalate over iron(III) oxalate in detail on pp138-139, and
these reasons apply in general, across all the iron-based processes.
There are also some remarks to this effect buried in my webpages, see:

There is an attempt to explain the differing photochemistry of the
two substances here:

and the role played by the ammonium ion in particular is suggested here:

having cited all this, I'll address your specific question about
replacing iron(III) oxalate by ammonium iron(III) oxalate in Kallitype:

First, concentrations. You're right: about twice the weight of the
latter in the same volume will give nearly the same iron(III)
concentration as the former. Actually slightly more: the factor is
about 2x 428/466 = 1.84, but I wouldn't be fussy. Ammonium iron(III)
oxalate is very soluble and easily dissolved as you will know.

The problem with the Kallitype process - and the reason why ammonium
iron(III) oxalate is not universally recommended for it - is the
precipitation of the very insoluble silver oxalate, which is greater
with ammonium iron(III) oxalate, because it generates more free
oxalate ion. Some folk don't mind spreading a thick suspension of a
solid onto their paper as a sensitized coating, but I do, and I
cannot believe it is conducive to good print quality. My philosophy
of siderotype is that the sensitizer should be a true aqueous
solution, to penetrate the cellulose fibres, and it should not hold
any solid matter in suspension, which will just lie on the paper
surface, and probably wash off.

Using iron(III) oxalate in Kallitype, it is possible to suppress the
precipitation of silver oxalate altogether - at least for a time - by
adding excess iron(III) salt such as the nitrate - see the
experiments described in Dick Steven's book. With ammonium iron(III)
oxalate you will always get a precipitate. I have tried quite a
number of ways to prevent this - without success. The first person to
do it will earn my respect and undying thanks! I am a great fan of
ammonium iron(III) oxalate, and use it wherever I can (e.g. my New
Cyanotype etc) but for iron-silver process I have had to compromise,
and use the less satisfactory ammonium iron(III) citrate in my
Argyrotype process. If I could have used ammonium iron(III) oxalate
without precipitating silver oxalate, I would have.

I think that about summarises my thoughts. You are welcome to quote
and publish this response on the APUG forum, if you think it will
help others. I don't think I'm subscribed to do this myself.

All good wishes, and let me know how you get on,