As placoderm sisters, is there an armored catfish?

Yes, but by convergence.
Adding the kwi kwi (Hoplosternum littorale, Figs. 1-4) to the large reptile tree (LRT, 1510 taxa) nests it with a derived catfish with (ironically) fewer bones, Ictalurus, not the one more closely related to the whale shark and placoderms, Clarias.

Hoplosternum littorale (Hancock 1828; 24cm length) is the extant kwi kwi, a South American catfish armored from nose to tail. Lacking teeth, the maxilla is reduced to a vestige near the premaxilla, far from the mandible joint. The in and out nares on the snout are enormous. Though armored it is not as closely related to placoderms as is the more primitive walking catfish, Clarias.

According to Wikipedia,
“Immature fish feed primarily on small aquatic crustaceans, especially Cladocera, Ostracoda, Copepoda, and Eubranchipoda. Chironomid larvae are important items in the diets of both juveniles and adults. The adult diet is dominated by mixed detritus, terrestrial insects, microcrustaceans, and aquatic beetles during the dry season, and mixed detritus and chironomid larvae during the wet season.”

Armor in adults interferes with dermal respiration,
so the intestines provide some fraction of the total respiration, as in aquatic turtles.

Enlarged nares, both in and out
(Fig. 4) served as anchors for the barbels that are characteristic of catfish. Note the skeleton of Hoplosternum, though heavily armored, shows no trace of a barbel, as in the catfish-like placoderm, Menaspis, now further separated from catfish due to the inclusion of taxa.

According to genomics,
(Betancur et al. 2013. 2017; Hughes et al. 2018) catfish should nest with knifefish, including Electrophorus, the electric eel, within the Actinopterygii.

In the LRT,
catfish nest with a previously untested clade, Placodermi, developing ray-fins by convergence. Prior cladograms suffered from taxon exclusion. They assumed ray fins were a monophyletic trait, that lobe fins were the outgroup and did not test sharks and ratfish, nor did they test placoderms and spiny sharks.

Try to never assume.
Keep adding outgroups until you are sure you need go no further. The base of any cladogram is where the most trouble is usually found.

References
Betancur-R R, et al. 2013. The tree of life and a new classification of bony fishes. PLoS Curr 5:ecurrents.tol.53ba26640df0ccaee75bb165c8c26288.
Betancur-R R, et al. 2017. Phylogenetic classification of bony fishes. BMC Evol Biol 17: 162.
Hancock J 1828. Notes on some species of fishes and reptiles, from Demerara, presented to the Zoological Society by John Hancock, Esq., corr. memb. Zool. Soc. In a letter addressed to the secretary of the Society. Zoological Journal, London v. 4: 240-247.
Hughes LC et al. (20 co-authors) 2018. Comprehensive phylogeny of ray-finned fishes (Actinopterygii) based on transcriptomic and genomic data. PNAS 115(24):6249–6254.
www.pnas.org/cgi/doi/10.1073/pnas.1719358115