ZHU Controller described a loss of separation event when issuing a descent clearance through an occupied altitude; the reporter listing new procedures; inadequate training; staffing and procedural design as casual factors.

Narrative

This was my first day working PSX Hi with 5 new arrivals into the Houston terminal area and new departures out of the SAT terminal area - I had that on my mind. I feel that the training was inadequate for this alone. Controllers were only required to run one 'DYSIM' problem. I was descending Aircraft Y from FL430 to FL270 to get him underneath Air Carrier Z at FL280; but keep him above Air Carrier W at FL260. He was also Negative RVSM which was occupying my thoughts. Air Carrier W was on the Sector 58 Frequency. I issued Air Carrier X descent to FL260 to get him underneath Air Carrier Z and place him right for direction. When I did so I completely overlooked Air Carrier W. Air Carrier X was descending almost right on top of Air Carrier W. Conflict Alert activated and I immediately told Air Carrier X to maintain FL270. The pilot stopped the plane pretty quick. I think they saw the Air Carrier W traffic visually; via TCAS; or both. I believe the combination of new arrivals and departures coupled with a Non-RVSM aircraft and some unusual situations caused me to overlook the traffic. I also feel that our specialty staffing is at a point where it is too low! Overtime is being assigned however I believe it is inadequate and we need more! I guess there are multiple safety issues here. [We had] inadequate training on the new procedures and inadequate staffing. I feel pressure to call for a D-Side only at the last minute. Also; the HUBEE1 SID out of SAT terminal area is poorly designed; in my opinion. It overlies the HAMUU1 arrival into IAH and increases workload when vectoring. I think when I get more used to the new procedures I will be more comfortable. In the mean time I will maintain extra vigilance and call for a D-Side more readily at the sacrifice of the 'break list'.