Hard to be completely sure. Lead Acid Chemistry is usually charged with a constant voltage source that may or may not be temperature compensated. As long as you remove the charger on a timely basis (don't leave it connected for days/weeks) you should be OK.
There are two things that are bad for SLA's: Running them competely 'flat' is usually fatal, and prolonged overcharging is also bad news. It converts electrolyte to gas, and SLA's don't have much electrolyte to start with. Generally you can charge lead-acid chemistry for quite a while at .05C (350ma for a 7Ah cell) without doing much damage. .5A is closer to .1C, so long term use of the charger isn't recommend, but if you have the charger on a timer, and limit the run time to about 12 hours, you will probably be OK.
Most SLA's fall into one of two categories. They either tolerate overcharging reasonably well, or they tolerate deep cycle operation reasonably well, but I don't think any SLA's do both. The SLA's you find in UPS's and providing battery backup are usually the ones designed to tolerate overcharging, not deep discharge.
The real question is whether or not your 12V/.5A charger has regulated voltage output.
If it is regulated to 12V, it will never fully charge a 12V Lead-Acid, that usually requires something around 14 volts. My advice would be to connect the charger in series with an meter and measure the actual charging current in operation. If the charging current gets below about .02C (about 140ma on a 7AH battery), you may never be able to reach full charge. Many of these 'wall warts', are NOT regulated, and have open circuit voltages of around 15 volts, and rely on the battery charging circuits to regulated the output.
Your NiCd/NiMH charger is more along the lines of constant current, and may depend on one or more of several different measurements to determine when full charge is reached (none of which your SLA's will activate except perhaps the 'time out'.
