oh, i see.
well, yes, pools are a single node (depending on the pools setup maybe 2, or 3 nodes).
workers, the mining-clients that are connected to the pool, are no nodes at all,
they are just some kind of bruteforcing tool that's hashing some data sent to them by the pool-server.
those workers themselves are in no way connected to the bitcoin-network, they are only connected to the pool-server.
the pool-server then is connected to one, or more bitcoin-nodes.
from a security point of view, pools nodes aren't any different from other nodes,
everyone is treated equally, they either follow the bitcoin-protocol-rules and everythings fine,
or they create their own rules and so create their own network, because everyone else will just ignore them.