tank height is fine , if you use floating plant cover or plant that reach the surface it will make them feel more secure and sometime will just hang out among the plants , ALL snakehead will eat live food but if they are accepting frozen bloodworm that is a good thing and i would recommend you try and keep them on a variety of frozen foods and or fresh food i.e shrimp pieces of fish etc this is much better for them than feeding feeders and a lot cheaper , honestly you will be glad in the long run.
what i have found is that they will probably be fine with any open water swimming snakeheads (micros pleuros maluroides) as tank mates and dont really recognize them as a threat , so your redlines at the same size should be fine , HOWEVER just use extreme caution as if it took there fancy it takes very little for them to kill each other ,sit and watch there behavior to each other ,any sign of nastiness split them up straight away ,also your redlines are going to outgrow the pulchra and very quickly so if all are the same size you probably only have a matter of weeks before the reds are far bigger than the pulchra ,another downside is that reds are extremely aggressive feeders and will eat all the food put in the tank so you need to make sure the pulchra are getting food, i keep two of the dottom dwelling snakeheads together but only one of each species and they are in a 9ft tank and even then only just tolerate each other ,i would not advise mixing different species of bottom dwelling snakeheads , your pulchra are likley to kill any newcomers pretty sharp , if you are determined to keep two types of sh i would strongly advise you chose pleuro Rather than red (which will out grow your tank in months anyway) for your open water swimmer and start them out smaller than the pulchra. never tried them with rays so cannot comment on this . by the way yes it is expected for them to be shy ,in fact them bottom dwelling snakeheads are quite shy always and like to hang about most the time in caves or among leaves etc. totally opposite to the water swimmers who are bold as brass and never stop moving
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