There's an excellent PDF worth reading about pipefish in public aquaria. They can be kept successfully if you know what you're doing, but they are fairly difficult fish. Your main problems are:
Identification: Some are truly freshwater, but most "freshwater" pipefish are in fact brackish water species. Very little is published to help you ID your specimens.
Food: At least initially, assume live-food only. Brineshrimp will work, but have almost no nutrition. Daphnia much better, especially if you get the daphnia well fed on algae before use. Some pipefish are more benthic than others, and these bottom dwelling species will take bloodworms and tubifex. Livebearer fry are eaten readily. Frozen foods are an option (eventually) but you main task is getting the pipefish feeding at all, then worry about weaning them onto alternatives.
Competition: they don't compete with other fish well. Keep alone, in groups of their own kind, or possibly with species that will ignore midwater live foods (wrestling halfbeaks and gobies would be possible, and perhaps flatfish, too).