Hold Person works on any PC polymorphed into another form
The Hold Person spell targets PCs regardless of state, and changing form via Polymorph Self or Polymorph Other does not change their status of being a person. This is how older editions work and most retroclones.
PCs changed by Polymorph Self can cast spells
BFRPG specifically says spells can be cast. “If the form assumed is capable of speaking and making appropriate gestures (as determined by the GM) the caster may use their own spells in the assumed form.“ Most older editions and retroclones work this way, too, with the exception of B/X which explicitly disallows it.
PCs changed by Polymorph Self don’t get elemental immunities or resistances
The BFRPG description of Polymore Self says “They gain the Armor Class and all physical attacks possessed by the form but does not gain any special, supernatural or spell-like abilities.” Only dragon breath is given as an example of a special ability. However, in the rule book, asterisks are placed after HD to indicate special abilities (and bonuses to XP). Most dragons have two, one for breath and one for immunities. Other text in the monster section says “The GM must calculate the XP award for any specific dragon as given on page 49, adding a special ability bonus (i.e. a “star”) if the dragon casts spells.” Immunities are called out in the monster description (e.g. “immune to normal cold, and take only
half damage from magical cold or ice”). Therefore, immunities are special abilities.
Furthermore, no other edition grants immunities until 5e, which doesn’t really have Polymorph Self.