How long are you planning to keep this tank running? I ask this because most people don't take longevity into consideration when mixing cichlids. For many of the experienced hobbyists, success of a mixture is based on co-inhabitating for AT LEAST 2 years. A lot of the 'my mix worked' on online forums is based upon several months together - but not over 2 years.
In 2 years time, basically all cichlids have time to reach sexual maturity - which is when their true temperaments and behaviors come out. A Jack Dempsey and Oscar can survive with rift lake cichlids - it does happen. However, the success rate is far outweighed by the failures. The big American cichlids are called tankbusters for a reason - they NEED space. A 125g is good for the two, but not much more.
Besides space, you need to take into consideration the dietary needs of your fish - your fish have a much different diet than many of the rift lake cichlids.