If the background colour clashes with a colour of the logo use the correct monochrome version.
If the logo has to be small, like less than 50 px / 40 pt / 1,4 cm / 0,6 in, either use only the typography or the squared logo.
You can modify the border radius of the square logo, even up to a full circle.
It's best to leave a space around the logo of 150% of the area around the logo. But a minimum of 120% is also enough.
The fonts used in the Simutrans logos are, Roadgeek 2005 Series EM for the word 'Simutrans', and Roadgeek 2005 Blend E5W for the word 'Nightly'. All these fonts are available in the Roadgeek package for download at MikeTheActuary.