/ɡ/ vs /ɣ/ in Frisian
You have it backwards. [ɡ] is an allophone of /ɣ/ that appears only at the beginning of a stressed syllable. Elsewhere, [ɣ] appears.
Ok, it does have âlden: http://gtb.inl.nl/iWDB/search?actie=article&wdb=WFT&id=727&lemma=%C3%A2lden&domein=0&conc=true . However, it considers it a lemma in its own right and not a plural of âlder. It appears that âlden has additional meanings, such as "elderly people" and "ancestors". If Dutch grammar is any indication, it is a substantivised form of "old" and thus means "old ones", while âlder is originally a comparative "older".
Even more conflictingly (and interestingly), the West Frisian Wikipedia also includes âldelju, so there seems to be a kind of three-way split between the -s suffix (which would usually be used with a word that ends in -er), the -en suffix, and the -lju suffix (also used with man and frou). I guess it's up to you on how you want to treat these forms.