I think the solution is to use subtype group layers.
I used the Water Distribution Simple Network Map to test with and got good results. It is contained within the Water Distribution Utility Network package you can download here. Remove all of the subtypes you don't want and then make all of the fields visible in the layer. If not, analyze will throw errors. I tested mainly on service connections, but I think the same would hold true for fittings. Just set a def query and remove the asset types you don't need.
I think there is an equivalent in Sewer==Sewer Simple Network. The asset group is still a coded value but the end user can just select the asset group as their initial option to collect. I tested in Collector and it worked well.
I haven't tested default values yet, but I would imagine they would be tied directly to the field itself --I would imagine the subtype would be honored. I don't THINK the edit feature template would be honored. Try setting a default value directly on the field and see if it propagates over to Portal. Just stop your service to free locks, and update a default value.
Finally, if you want to set symbols yourself, to the right of the add data button is the Add Preset | Subtype Group Layer. Use it to add data directly from SDE, then symbolize accordingly.
I hope this helps!