"Shorting" clock inputs of multiple flip-flops
Inputs can be connected together, usually at least 10 are okay when driven from a logic output in the same family. 

They should be connected to exactly 1 output (or to something such as a pullup resistor which simulates an output). 

The LM555 (bipolar version) is advertised as being TTL-compatible. It is guaranteed to have less than 350mV drop when sinking 5mA with a 5V supply. Each standard TTL input is 1.6mA so it can drive as many 3 inputs with more than 400mV of noise immunity (800mV or less is recognized as a 0 by the flip-flop). 



A fan-out of four will almost surely work okay for a hobby project. If you want to be really sure, drive a single TTL gate input with the 555 and use the output of the gate to drive the clock inputs. 

Do not put resistors in series with TTL inputs. The resistors will harm the noise immunity when at logic '0' or may even keep the input from being recognized as a '0'. Sometimes you will see low value (some tens of ohms) resistors in series with clock signals to help tame ringing but a 555 is rather sluggish (100ns rise/fall times) so I doubt that will be an issue.