When I inject signals on a single res pitch toy I use a SPST sw to allow removing the osc completely. You can still mod a single res pitch toy with a pot or body contacts and be able to inject a signal.
For injecting a signal you can use any of the osc I am talking about. For single res pitch toys I prefer 2 555s, the second to modulate the first. That way there is some variety. I use a SPST to be able to remove the second 555 all together. No matter which osc type you choose you will want to limit how fast the signal is to make the toy stable.
For toys with coil/xtal/resonator I use the LTC1799 or sometimes a 2 inv osc. I find the LTC1799 best because of its range, but a pain because of its size. These toys tend to operate in the 2Mhz and up range. I probe the two traces the coil/xtal/resonator are connected to to see if one is better/more stable than the other. Again I use a SPST to control injecting the signal.
In addition to injecting on the clock/pitch of a toy you can probe other parts of the circuit, avoiding the power regulator areas.