The Direct Commission Engineer (DCE) Program is normally for Naval, Mechanical, Civil, or Electrical Engineers (boat types). We do have Aeronautical Engineers, but they normally come from the ranks of the junior pilots and typically get trained to fill a role like the Maintenance Officer (MO) for the other services. That is not saying that you can't get in under the DCE program and go into aviation.
The Direct Commisison Aviator (DCA) program is for military pilots requesting transfer into the CG. Highest rank coming in on this program is LTjg (O2).
We do acknowledge the Inter-service transfer as a possibility, but our manuals refer to it in a CG to other service example. For a C-130 rated pilot, I would try and pursue the inter-service transfer. Your rank follows you. The CG has stayed away from this mainly I believe due to the costs associated with accepting a retirement cost with only 10-12 years of CG service. Local Recruiters are not going to be knowledgeable on Inter-service transfer so I would call the CG Recruiting Command and get on the phone with a civilian at the command. They are the true experts and can help the best.
If you come into the CG on the DCA program, you can expect to serve in 2-3 operational flying units and 0-1 Standardization/Training units prior to retiring as an O-4. Typically, fixed wing flying opportunities dry up at the O-5 level.
As far as transitioning into the C-130, the best you can get out of our transition course is FP and it is dictated by the time out of the C-130 and your flight performance. If you transfer directly in from a std Herc (all but the J) as an AC/IP with no lapse in quals, you can expect to leave the transition course as a LSCP or FP and and quickly (6 months to a year) upgrade to AC. Like cgflier said, if you are a J pilot or transition into the CG and get J's, then you can expect E-City, NC for 4-8 years. The J models will all be in NC and the Standardization Team will also be stationed in E-City but will be attached to Mobile. The C-130H Stan team is attached to Mobile, AL, but is stationed in Clearwater, FL. Currently, transitions into CG C-130's are CAE in Tampa, FL for the H model; and Keesler AFB for the J model.
Hope this helps.