Here is an answer from The Star-K.
Similarly fish gelatin in order to be considered kosher must be produced from kosher species of fish. The use of fish gelatin with meat foods poses an interesting question. As we have mentioned the Shulchan Aruch (Yore Deah:116) prohibits cooking meat and fish together because of health concerns. When dealing with possible health concerns we are more stringent than with possible Issur (prohibited substances). For this reason there is a question among the commentaries if the nullification in sixty rule applies to unhealthy substances as it does with prohibited substances. The custom is that one can nullify unhealthy substances in sixty (see Nekudas Hakesef, Yoreh Deah:116 & Pische' Tshuvah).
Today there are many reasons for leniency in the use of fish gelatin
together with meat. Many rabbinic authorities are of the opinion that
the nature of some foods have changed, making the mixture of meat and
fish no longer unhealthy (see Magen Avrohom Orach Chaim 173:1, Tshuvos
Chasm Sofer vol:1 #101) In addition there is reason to say that not
all fish are dangerous with meat. It may be that only the type
mentioned in the Gemora (Binita) is unhealthy (see Pische' Tshuvah,
Yoreh Deah 116:3). There is also good reason to say that the unhealthy
aspects of fish cooked with meat are found in the flesh of the fish,
not in their skin and bone (from which gelatin is made). Gelatin does
not have fish flavor. As such it may not harbor the harmful effects
that fish carry (see Pische Tshuva ,Tshuvos Sride Eish vol:2 #67 re.
cooking beef in fish oil). With this same reasoning we can say that
gelatin can be batel (nullified) with a majority of other food
ingredients and can be mixed with meat. (As stated according to R'
Aharon Kotler, zt"l regarding animal gelatin & milk) As a result of
these reasons it is acceptable to use fish gelatin with meat. We may
use much the same reasoning in the reverse case, to allow animal
gelatin with fish.
Note that the language here is based in "leniency" and "it is acceptable" so this is not an unequivocal endorsement. CYLOR.