Placidus is a house system. It determines the house cusps. Planets might fall into different houses using different house systems but their sign and degrees stay the same. Orb is how far apart the two planets involved are from being exact for the aspect indicated. For example- an opposition is 180 degrees. If you have two planets that are 183 degrees from each other; they would be planet A opposite planet B, orb 3 degrees. Most of the orbs lists look fine except two. I think Mars conj Jupiter and Jupiter conj Midheaven is a bit too wide. I would use 5 degree orb for conjuncts unless the Sun or Moon are involved. Others will disagree. Both things you asked about are personal preferences. You could ask two separate questions about house system preference and orb preferences if you like. I’d include a why in the questions.