eh...spikey and fast enough are all results of ur human logic. this might come as a surprise, but guess wut, ur aro doesn't have the power of reasoning. they'll just try to swallow anything they think fit in their mouth, and when ur sliver catches ur dats (and trust me, the dats are NOT fast enough), and starts to swallow 'em, those spikes on dats' dorsal fins are gonna stuck in the sliver's throat making it a lose lose situation -___-.
this mite not happen, but it is a risk. so much for me to think that my jar is smart enough not to swallow a royal pleco... -____________-