I've never had my ones with Corydoras, they currently share with four ~9cm Hoplos; one ~7cm Hypoptopoma spp.; one ~7cm Humphead Glassfish; nineteen(?) 1-7cm Rabbit Snails; one ~7cm Golden Wonder Killifish; one ~8cm African Butterfly Fish (last two stick to the calmer sections of surface water). They did share with my group of nine ~3-7cm Synodontis nigriventris for a while last year and things were fine once there was enough hiding places for everyone in my 48x12x15, plus my tiny ~4cm Microsynodontis polli is still around.
Chances are they would mix fine with Corydoras, I'd be very surprsied if they bothered the Harlequins and Pyjamas can take the "heat" of Lionhead Cichlids, so I'd imagine typical "all bark no bite" behaviour of Firemouths would not bother them at all.
Try and get at least four (similar size would help, I bought wildly different sizes for just £25 for the whole group), possibly five. One of the major identification marks of true Pyjamas are that all barbels are branched, most synos have unbranched outer barbels.