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If you don't want the blooms, it will not harm the plant at all to prune them before they open. Pruning guidelines for shrubs are normally made to help maximize blooming, but if blooming is not a concern, you can prune most shrubs anytime between early spring and late summer without harm to the plant.

About 3 1/2 to 4 foot spacing should be sufficient.

Yes, it could be starts from the plant, either from seed or from suckers. If they are growing on their own, you can dig them up and transplant them.

Yes, if they look hagard, go ahead and cut them back, though normal pruning of photinia usually takes place anytime between early spring and early fall--late spring, early summer being the best time. Unruly/unhealthy plants can be cut back 6 inches to the ground and will come back--annual pruning will help keep them maintained and manageable. They can take a pretty severe pruning, so you can cut them back to a manageable size without harming them.

Some people recommend using a fertilizer higher in phosphorous while other say a balanced fertilizer is the way to go. I would probably go with a slow-release, balanced fertilizer for optimal results. An even fertilizer (8-8-8) would be good. Fertilize once in spring and once in early fall.

By clipping, I assume you mean take cuttings from them. This can be done during routine pruning, which is anytime between spring and fall (late spring/early summer is ideal). These articles should also help: https://www.gardeningknowhow.com/ornamental/shrubs/photinia/growing-red-tip-photinia.htm, https://www.gardeningknowhow.com/ornamental/trees/tgen/how-to-root-cuttings-from-various-shrubs-bushes-and-trees.htm