The SSIDs by themselves are not security issues.
Where it becomes more of a concern is if these devices have well known default passwords. That gives passersby access to the device.
Even that is not, necessarily, a security issue. Unless that access can be taken advantage of to gain access beyond the device. Or, used to install nefarious software on the device that can be controlled remotely, not through its own Wi-Fi server (very short range.)
As R.Proffitt says, part of the advantage of having all those devices on your network is that they can talk to each other, and your computers and phones can talk to them. Your local Wi-Fi is intended to be walled off from the rest of the Internet, partly in order to give you a safe place for your devices to communicate. If you move some of them to another network, then you lose most of the advantages of even owning those devices.
IoT devices, as a general rule, have a fairly poor reputation for security. I'm sure that some are secured well. But, a lot of name brand models have known vulnerabilities. If security is important to you, then you're going to need to thoroughly research each model for those vulnerabilities. Those you find, you should patch, if one is available. Or, remove them from your network. Some that are not even on your Wi-Fi, but are talked to by your phone, computer or other devices that *are* on the network could have vulnerabilities that can give a hacker access. Those should be patched or removed from your home. And, you should research any new devices that you're thinking of adding to your system. And, routinely do check-up searches for new vulnerabilities on devices that you previously found to be clean.
Yes, that's a huge amount of work. But that's where we're currently at with IoT security. Until a large percentage of people start voting with their wallets, security is going to remain a superficial afterthought by the manufacturers.
Having said all that. Your actual vulnerability will also depend on where you live.
If a given device does not, itself, have an Internet-facing interface, then you're probably fairly safe. IOW, if you have not set up a port forward or similar access on your router, then your devices are only vulnerable to really short range access. Unless a lot of traffic passes nearby. Unless you live right next to a university dorm or other concentration of bored, smart people. Unless you live in some particularly densely populated spot. Unless one of those, chances are you're not going to be within range of anyone looking for devices to penetrate. If you live in a rural area, or in a spread out 'burb away from the main streets, then it's not really worth anyone's time to go seeking those dispersed devices.
Up to you to weigh your own comfort.
And, I will also reiterate another suggestion from R.Proffitt. Where possible, turn off the access systems you don't use. So, if you're not using your printer's Wi-Fi server to print on-the-fly, then turn it off. Same with the WeMo. Anywhere you can, if it's an ancillary connection point that you don't use in your routine use of the product, turn it off.