user15159: I was wondering how I could overcome the next issue;
My ESP8266 12E is connected as follows:

GPIO0 ground if needed to program, vcc otherwise
GPIO2 vcc
GPIO15 ground

Bootmodes explained in the this schematic
I also want to connect a MCP3002 to my ESP. However to use SPI I also need the GPIO15 pin to be connected to the Chip Select pin. 
MCP connections explained in this article

I tried connecting the GPIO15 pin from the ESP to the CS pin of MCP3002 and connecting ground in between. This did not work though, should I switch/remove the ground after boot somehow?
(Or should I connect it differently?)

user37622: The io 15 is SS if esp8266 is slave. If esp8266 is SPI master, you can choose any free pin as CS of the slave.

The esp8266 arduino boards package SPISlave library examples have a solution for master if esp8266 is slave.
The state of the boot pins is required only at boot. You should wire boot pins over a 10 kOhm resistor to create a pull-up/pull-down which can be override by pin output state to use the pins for devices which are OK with or need pull-up or pull-down, for example momentary switches (buttons).

15159: <p>I was wondering how I could overcome the next issue;</p>

<p>My ESP8266 12E is connected as follows:</p>

<ul>
<li>GPIO0 ground if needed to program, vcc otherwise</li>
<li>GPIO2 vcc</li>
<li>GPIO15 ground</li>
</ul>

<p><a href="https://www.circuito.io/blog/programming-esp8266-12e/" rel="nofollow noreferrer">Bootmodes explained in the this schematic</a></p>

<p>I also want to connect a MCP3002 to my ESP. However to use SPI I also need the GPIO15 pin to be connected to the Chip Select pin. </p>

<p><a href="https://prkl.info/esp8266-mcp3002-driver/" rel="nofollow noreferrer">MCP connections explained in this article</a></p>

<hr>

<p>I tried connecting the GPIO15 pin from the ESP to the CS pin of MCP3002 and connecting ground in between. This did not work though, should I switch/remove the ground after boot somehow?
(Or should I connect it differently?)</p>
 37622: <p>The io 15 is SS if esp8266 is slave. If esp8266 is SPI master, you can choose any free pin as CS of the slave.</p>

<hr>

<p>The esp8266 arduino boards package SPISlave library examples have a solution for master if esp8266 is slave.</p>

<p>The state of the boot pins is required only at boot. You should wire boot pins over a 10 kOhm resistor to create a pull-up/pull-down which can be override by pin output state to use the pins for devices which are OK with or need pull-up or pull-down, for example momentary switches (buttons).</p>
